1. Technical Field
This invention relates generally to optical relays for head up displays and the head up displays employing such optical relays, and more particularly to an optical relay having reflective properties at certain angles of incidence relative to surfaces of the optical relay and transmissive properties at other angles of incidence relative to the surfaces.
2. Background Art
Head up displays are typically deployed in vehicles such as automobiles or aircraft to allow the driver or pilot to view information projected by the display without requiring the driver or pilot to divert his view away from the outside environment. Head up displays employ optical relays to direct light from a projection source to a viewer's eye. The optical relay design for head up displays typically magnifies the image generated by the projection source, such as a scanning display, liquid crystal display (LCD), or other projection source. The optical relay then places the magnified image such that it is optically perceived at some distance outside the vehicle or aircraft. For example, in some applications the head up display will present an image that appears to the viewer to be, for example, around one and a half meters away from the operator and outside the vehicle or aircraft.
To provide the proper magnification, the optical relay can require a sufficient optical path length. A certain amount of minimum length can be needed in some applications for the optics within the head up display to suitably magnify the image. This needed optical path length can provide a design constraint on the physical size of the head up display, in that the form factor of the head up display must be larger than may be desired to achieve the proper magnification of the projected image. Reduction in the overall size of prior art head up displays can be further limited by other optical requirements including eyebox size, field of view, and/or distance of apparent image location.
There is thus a need for an optical relay and head up display capable of delivering adequate optical path length in a reduced form factor.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions or relative angles of some of the elements in the figures may be exaggerated or repositioned relative to other elements to help to improve understanding of embodiments of the present invention.